Channel sounding techniques for a wireless communication system

ABSTRACT

A technique of operating a wireless communication system includes determining respective geometries of multiple subscriber stations, which include a first subscriber station and a second subscriber station, with respect to a serving base station. Respective channel sounding bandwidths for sounding the channel between the multiple subscriber stations and the serving base station are then scheduled, based on the respective geometries. The respective channel sounding bandwidths include a first channel sounding bandwidth (associated with the first subscriber station) and a second channel sounding bandwidth (associated with the second subscriber station). The first channel sounding bandwidth is greater than or equal to the second channel sounding bandwidth and the first subscriber station has a lower geometry than the second subscriber station.

BACKGROUND

1. Field

This disclosure relates generally to channel sounding and, morespecifically, to channel sounding techniques for a wirelesscommunication system.

2. Related Art

In general, orthogonal frequency division multiplexing (OFDM) systemssupport high data rate wireless transmission using orthogonal channels,which offer immunity against fading and inter-symbol interference (ISI)without requiring implementation of elaborate equalization techniques.Typically, OFDM systems split data into N streams, which areindependently modulated on parallel spaced subcarrier frequencies ortones. The frequency separation between subcarriers is 1/T, where T isthe OFDM symbol time duration. Each symbol may include a guard interval(or cyclic prefix) to maintain the orthogonality of the symbols. Ingeneral, OFDM systems have utilized an inverse discrete Fouriertransform (IDFT) to generate a sampled (or discrete) compositetime-domain signal.

Wireless networks have generally used an estimated received signalstrength and an estimated carrier to interference and noise ratio (CINR)of a received signal to determine operational characteristics of thenetworks. As one example, IEEE 802.16e compliant mobile stations (MSs)are required to estimate a received signal strength indicator (RSSI) anda CINR of a received signal. The RSSI associated with a serving BS maybe used by an MS for cell re-selection and the CINR, which is reportedto the serving BS, may be used by the serving BS to adapt a downlinktransmission rate to link conditions.

Accurate reported CINRs are desirable, as inaccurate reported CINRs mayimpact performance of a wireless network. For example, reporting a CINRthat is above an actual CINR may decrease network throughput due toframe re-transmission, while reporting a CINR that is below the actualCINR may cause the serving BS to schedule data rates below a supportabledata rate. According to IEEE 802.16e, RSSI and CINR estimates at an MSare derived based on a preamble signal, which is an orthogonal frequencydivision multiple access (OFDMA) symbol that is transmitted at thebeginning of each OFDMA frame.

Similarly, wireless networks that employ third-generation partnershipproject-long term evolution (3GPP-LTE) compliant architectures arerequired to employ uplink reference signals (RSs) for uplink CINRestimation, which is used by the network to schedule uplink transmissionfor user equipment (subscriber stations (SSs)). Respective sequences ofthe RSs are used to uniquely identify an SS and, when transmitted fromthe SS to a serving base station (BS), may be used by the serving BS inchannel characterization. A known channel sounding (channelcharacterization) approach has proposed limiting a channel soundingbandwidth of cell-edge SSs, i.e., SSs operating at or near an edge of acell, to reduce interference with neighboring cells and to improveuplink CINR estimation. In this approach, cell-edge SSs sound a portionof a system bandwidth in one sounding symbol and employ frequencyhopping to cover the entire system bandwidth using multiple soundingsymbols. Following this approach, non-cell-edge SSs are allowed to soundthe entire system bandwidth with a single sounding symbol.Unfortunately, the above-described approach generally increases systembandwidth requirements (due to increased scheduling overhead), resultsin increased inter-cell interference (due to higher power spectraldensity (PSD) associated with narrower bandwidths), and does notgenerally improve channel estimation accuracy.

What is needed are techniques for improving channel sounding in awireless communication system.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and is notlimited by the accompanying figures, in which like references indicatesimilar elements. Elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale.

FIG. 1 is a diagram of an example uplink (UL) subframe that includes ademodulation reference signal (RS) positioned in a fourth (middle)symbol of each of two slots.

FIG. 2 is a flowchart of a channel sounding bandwidth assignment processthat may be, at least partially, employed in a scheduler of a wirelesscommunication system, according to the present disclosure.

FIG. 3 is a flowchart of a process for receiving/transmitting anassigned channel sounding burst (at a scaled target transmit powerspectral density level) at/from a subscriber station (SS), according tothe present disclosure.

FIG. 4 is a flowchart of a carrier to interference and noise ratio(CINR) determination process that may be employed in base stations (BSs)of a wireless communication system, according to another embodiment ofthe present disclosure.

FIG. 5 is a block diagram of an example wireless communication systemthat may assign channel sounding bandwidths to SSs according to variousembodiments of the present disclosure.

FIG. 6 is an example graph that provides a comparison of errorperformance of a wireless communication system that performsconventional channel sounding with a wireless communication system thatperforms channel sounding according to various embodiments of thepresent disclosure.

DETAILED DESCRIPTION

In the following detailed description of exemplary embodiments of theinvention, specific exemplary embodiments in which the invention may bepracticed are described in sufficient detail to enable those skilled inthe art to practice the invention, and it is to be understood that otherembodiments may be utilized and that logical, architectural,programmatic, mechanical, electrical and other changes may be madewithout departing from the spirit or scope of the present invention. Thefollowing detailed description is, therefore, not to be taken in alimiting sense, and the scope of the present invention is defined onlyby the appended claims and their equivalents. In particular, althoughthe preferred embodiment is described below in conjunction with asubscriber station, such as a cellular handset, it will be appreciatedthat the present invention is not so limited and may be embodied invarious wireless devices, e.g., personal digital assistants (PDAs),digital cameras, portable storage devices, audio players, computersystems, and portable gaming devices, for example.

As is used herein, the term “user equipment” is synonymous with the term“subscriber station,” which is used to denote a wireless deviceassociated with a wireless communication system. The term “channel,” asused herein, includes one or more subcarriers, which may or may not beadjacent. Moreover, the term “channel” may include an entire systembandwidth or a portion of the entire system bandwidth. As used herein,the term “demodulation RS” means an RS that is assigned to (andtransmitted by) an SS, received by a serving base station (BS), and usedby the serving BS for channel estimation. According to one aspect of thepresent disclosure, an uplink (UL) channel sounding bandwidth assignmenttechnique is employed that generally reduces scheduling overheadassociated with sounding a UL channel of a wireless communicationsystem. The UL channel sounding technique schedules low-geometry SSs(i.e., SSs with relatively low CINRs) to sound a channel using a channelsounding bandwidth that is greater than or equal to a channel soundingbandwidth of an SS having a higher geometry. The bandwidth for thelow-geometry SS may correspond to an entire system bandwidth, e.g., 5MHz or a portion of the entire system bandwidth, e.g., 2.5 MHz.

To support channel dependent UL scheduling and closed-loop powercontrol, it is desirable to sound an entire system bandwidth. Ingeneral, assigning a channel sounding bandwidth to a low-geometry SSthat is greater than or equal to a channel sounding bandwidth assignedto a high-geometry SS does not result in increased interference. Thatis, while a low-geometry SS transmits wider bandwidth sounding referencesignals (RSs) at lower power in the frequency-domain, when the soundingRSs are code division multiplexed (CDM) signals, losses in thefrequency-domain are gained back in the time-domain. Moreover, when theSS is a cell-edge SS, channel sounding over a wider bandwidth results inless interference in neighboring cells, as the cell-edge SS usuallydecreases a transmit power level when a bandwidth of a sounding RS isincreased. As noted above, conventional channel sounding schemes haveproposed limiting a channel sounding bandwidth of a cell-edge SS to lessthan a channel sounding bandwidth of non-cell-edge SS. A geometry of anSS with respect to a serving BS may be determined by, for example,determining a CINR associated with the SS. For example, power-limitedsubscriber stations (SSs), e.g., SSs that are at or near a cell-edge,may have a relatively low associated CINR at a serving BS and, thus, beclassified as a low-geometry SSs.

One technique for detecting a power-limited SS (e.g., a cell-edge SS)utilizes a feedback indicator, e.g., a 1-bit indicator, from the SS. Inthis case, the SS determines whether it is power-limited (e.g.,transmitting at maximum or near maximum power) and asserts the 1-bitindicator in a UL data or control message to report a power-limitedcondition to a serving base station (BS). Alternatively, multiple bitsmay be employed in a feedback indicator to provide an indication of anextent to which an SS is power-limited. Another technique for detectinga power-limited SS may use a carrier to interference and noise ratio(CINR) calculated, by the serving BS, on a received RS. In thisapproach, the serving BS calculates a CINR of an RS transmitted from agiven SS and compares the calculated CINR to a threshold (e.g., 3.5) todetermine whether the given SS is power-limited. According to anotheraspect of the present disclosure, a power-limited SS may be detected bya serving BS that employs a bandwidth provisioning technique. Accordingto the bandwidth provisioning technique, a scheduler assigns a firstbandwidth (e.g., one RB) to a given SS. The serving BS then communicatesan initial schedule (including an RS) to the given SS and determines aCINR on the RS transmitted by the given SS over the first bandwidth. Thescheduler then increases a bandwidth assigned to the SS to a secondbandwidth (e.g., two RBs). The serving BS then communicates a newschedule (including a new RS) to the SS and determines a CINR of the newRS transmitted by the given SS over the second bandwidth. The techniquemay be extended, e.g., to three RBs, four RBs, etc. In general, if theCINR drops at the wider bandwidth(s), a power-limited SS is indicated.

According to one embodiment of the present disclosure, a method ofoperating a wireless communication system includes determiningrespective geometries of multiple subscriber stations (which include afirst subscriber station and a second subscriber station) with respectto a serving base station. Respective channel sounding bandwidths forsounding the channel between the multiple subscriber stations and theserving base station are then scheduled based on the respectivegeometries. The respective channel sounding bandwidths include a firstchannel sounding bandwidth (associated with the first subscriberstation) and a second channel sounding bandwidth (associated with thesecond subscriber station). The first channel sounding bandwidth isgreater than or equal to the second channel sounding bandwidth and thefirst subscriber station has a lower geometry than the second subscriberstation.

According to another embodiment of the present disclosure, a method ofoperating a wireless communication system includes receiving (at a firstsubscriber station) a power control target, which specifies a targettransmit power spectral density level for transmitting a target channelsounding burst from the first subscriber station. The first subscriberstation scales the target transmit power spectral density level based ona first channel sounding bandwidth associated with a first channelsounding burst, which is a code division multiplexed (CDM) signal. Thefirst subscriber station then transmits the first channel sounding burstat the scaled target transmit power spectral density level. In thiscase, due to the fact that the first channel sounding burst is a CDMsignal, a serving base station can usually recover the first channelsounding burst (even when the SS is a power-limited cell-edge SS).

According to yet another embodiment of the present disclosure, awireless communication system includes a base station and a scheduler.The base station is configured to determine respective geometries ofmultiple subscriber stations with respect to the base station. Thescheduler is configured to set respective channel sounding bandwidthsfor sounding a channel between the multiple subscriber stations and thebase station, based on the respective geometries of the multiplesubscriber stations. In this embodiment the respective channel soundingbandwidths of lower geometry subscriber stations (included within themultiple subscriber stations) are selected to be greater than or equalto the respective channel sounding bandwidths of higher geometrysubscriber stations (included within the multiple subscriber stations).

With reference to FIG. 1, an example uplink (UL) subframe includes areference signal (RS) positioned in a fourth (middle) symbol of eachslot. In the illustrated example, a UL subframe includes two slots, eachof which include seven LBs and which each encode a symbol. It should beappreciated that the techniques disclosed herein are broadly applicableto UL subframes that employ more or less than the illustrated number ofLBs. In the example UL subframe depicted in FIG. 1, the RS in the middlesymbol in each of slots 0 and 1 are demodulation RSs. In general, ULreference signals (RSs) may take various forms, e.g., demodulation RSsand channel sounding RSs. A demodulation RS is associated withtransmission of uplink data and/or control signals. In contrast, achannel sounding RS is not usually associated with uplink datatransmission. Usually, a demodulation RS is used to estimate a ULchannel before decoding data transmitted on the UL channel. In thiscase, the demodulation RS has the same bandwidth as the data andoccupies the same set of subcarriers as the data. UL RSs may be based onZadoff-Chu (ZC) sequences, which are non-binary unit-amplitudesequences.

Typically, ZC sequences have ideal cyclic autocorrelation and, as such,ZC sequences are constant amplitude zero autocorrelation (CAZAC)sequences. Cyclic shifted versions of a ZC sequence have lowcross-correlation, which allows the impact of an interfering signal tobe evenly spread in the time-domain, after correlating the receivedsignal with a desired ZC sequence. In general this allows for morereliable detection of a desired channel. According to variousembodiments of the present disclosure, channel sounding RSs may bescheduled in any of the LBs in either slot of the UL subframe. Accordingto one or more embodiments, channel sounding symbols scheduled in a sameLB of a subframe are configured to be orthogonal when the channelsounding symbols are assigned to a same channel. That is, when multipleSSs are scheduled to transmit channel sounding symbols over the samechannel (i.e., group of subcarriers), the scheduled channel soundingsymbols for each of the multiple SSs are configured as code divisionmultiplexed (CDM) sequences. The CDM sequences may be generated bycyclic shift of one or more base sequences. In general, a length of thecyclic shift may be based on a typical time delay spread associated withthe SSs in a cell. For example, in a wireless communication systemhaving a typical time delay spread of five microseconds and a samplingfrequency of 7.68 MHz, a cyclic shift of forty may be employed. The CDMsequences may be, for example, CAZAC sequences, generated in a number ofways. The generation of the CDM sequences is not particularly relevantto the present disclosure and, as such, is not discussed further herein.

As previously noted, to differentiate SSs (and/or cells), multipleunique RSs may be implemented within a wireless communication system.Power-limited SSs may be indicated by SSs that are operating at or nearmaximum transmitter power (e.g., SSs with a transmitter power of 24 dBm(decibels with respect to one milliwatt)). According to various aspectsof the present disclosure, overhead for channel sounding may usually bereduced by determining respective geometries of multiple SSs withrespect to a serving BS. Channel sounding bandwidths for sounding achannel with a channel sounding symbol (or symbols) is then set basedupon the respective geometries of individual SSs with respect to aserving BS. For example, a channel for a high-geometry SS (e.g., an SSwith a CINR of about 15 dB) may be characterized based upon thetransmission of channel sounding symbols from the high-geometry SS overa channel sounding bandwidth of one resource block (e.g., twelvesubcarriers). As another example, a channel for a low-geometry SS (e.g.,an SS with a CINR of about 0 dB) may be characterized based upon thetransmission of a channel sounding symbol from the low-geometry SS overa channel sounding bandwidth of four resource blocks (e.g., forty-eightsubcarriers). As yet another example, a channel for a medium-geometry SS(e.g., an SS with a CINR of about 7.5 dB) may be characterized basedupon the transmission of channel sounding symbols from themedium-geometry SS over a channel sounding bandwidth of two resourceblocks (e.g., twenty-four subcarriers). In this example, it should beappreciated that the high-geometry and medium-geometry SSs are requiredto frequency hop to sound the channel covered by the low-geometry SS inone sounding symbol. It should also be understood that the CINRs, setforth above, are example CINRs.

In general, a length of an RS (r_(u)(n)) is determined by a length of adiscrete Fourier transform (DFT), e.g., a fast Fourier transform (FFT),that is used for the RS (i.e., the number of subcarriers employed). Forexample, when an RS is assigned one resource block (i.e., twelvesubcarriers in the frequency-domain), eleven basis sequences may begenerated using a cyclic extension approach, i.e., r_(u)(n), 0≦u≦10,0≦n≦NFFT-1, where NFFT is the size of the DFT. From each basis, twelveorthogonal sequences may be generated using a cyclic shift in thefrequency-domain. An uplink transmitter of an SS may implement one of aphase shift keying (PSK), a quadrature amplitude modulation (QAM), orother data modulation scheme, depending upon which modulation scheme isscheduled. It should be appreciated that any of the various PSK, e.g.,pi/2 BPSK, QPSK and 8-PSK, or QAM, e.g., 16-QAM and 64-QAM, modulationtechniques may be implemented in a wireless communication systemconstructed according to the present disclosure.

According to one or more embodiments of the present disclosure, aserving BS initially calculates a CINR of a training signal transmittedfrom a given SS, during a training sequence, and compares the calculatedCINR to one or more thresholds to determine a geometry of the given SS.The training signal may be, for example, a random access preamble or achannel sounding burst. In the event that the given SS is determined tobe a low-geometry SS, a channel sounding RS having a relatively widebandwidth may be assigned, by a scheduler (e.g., a network scheduler),to the SS. In the event that the SS is later detected to be at a highergeometry, the scheduler may assign a different channel sounding RS,having a narrower bandwidth, to the SS. It should be appreciated thatthe time period over which an SS is scheduled to transmit the channelsounding RS should generally be less than a coherence time of the ULchannel (i.e., a time over which the UL channel is stable). Moreover, abandwidth assigned to the channel sounding RS should include enoughsubcarriers such that code division multiplexing (CDM) can be employedfor the channel sounding RSs transmitted by the SSs (for example, twelvesubcarriers are typically required to implement CDM for the UL channel).The channel sounding symbols transmitted by the different SSs shouldusually be orthogonal, such that multiple SSs can transmit channelsounding RSs simultaneously over the same channel (group of subcarriers)without interference. A serving BS can then receive the respectivechannel sounding RSs transmitted by respective SSs and accuratelydetermine channel characteristics based on the received channel soundingRSs.

For power-limited SSs (e.g., cell-edge SSs that are transmitting at apower level of about 24 dBm) that are farther from the serving BS, achannel sounding bandwidth of a channel sounding symbol may be assigneda relatively wide bandwidth, e.g., an entire system bandwidth. Moreover,the sounding RSs may be utilized in conjunction with demodulation RSs toimprove accuracy of CINR calculations for the SSs. For example, when achannel sounding RS occupies a same channel as a demodulation RS, CINRsassociated with the demodulation and sounding RSs may be averaged toprovide a better indication of channel quality. To improve noise andinterference estimates, one or more blank cyclic shifts may be employed.That is, certain of the CDM sequences (blank cyclic shifts) may not beassigned to an SS. In this manner, a serving BS may estimate noise andinterference based on decoded blank cyclic shift(s).

Turning to FIG. 2, a process 200 for assigning channel soundingbandwidths to SSs is depicted. The process 200 may be predominantlyemployed in a scheduler, e.g., a network-based scheduler, of a wirelesscommunication system. The process 200 is initiated at block 202, atwhich point control transfers to decision block 204. In block 204, theserving base station (BS) determines whether a training signal has beenreceived from an SS. If a training signal is received in block 204,control transfers to block 206, where a CINR for the SS is determinedbased on the training signal. If a training signal is not received inblock 204, control loops on block 204 until a training signal isreceived. Following block 206, control transfers to decision block 208where it is determined, e.g. by a scheduler, whether the CINR is lessthan a threshold. If the CINR of the SS is less than a threshold,control transfers to block 212, where a channel sounding bandwidth forthe SS is set to a first bandwidth. If the CINR of an SS is not lessthan a threshold, control transfers from block 208 to block 210, where achannel sounding bandwidth is set to a second bandwidth, which is lessthan or equal to the first bandwidth. Following blocks 210 and 212,control transfers to block 214. In block 214, the serving BS transmitsthe channel sounding burst schedule to the SS. Next, in decision block216, the serving BS determines whether additional SSs require training.If so control transfers to block 204. Otherwise, control transfers fromblock 216 to block 218, where control returns to a calling routine.

A CINR of a received signal may be estimated through a number ofapproaches. As a first example, U.S. Patent Application Publication No.2006/0133260 discloses a channel estimation based approach forestimating CINR that isolates noise and interference components usingpilot sequences and estimates a channel power by subtracting a combinednoise and interference power estimate from a received power estimate. Asa second example, U.S. Patent Application Publication No. 2006/0093074discloses a difference based approach for estimating CINR that assumesthat adjacent pilot locations have the same subchannel characteristics.In view of this assumption, noise and interference components areisolated by subtracting adjacent received signals.

Moving to FIG. 3, a process 300, for receiving a power control target(having an associated target transmit power spectral density level,which has an associated bandwidth) and an assigned channel soundingburst schedule and transmitting an assigned channel sounding burst at ascaled target transmit power spectral density level, is illustrated. Inblock 302, the process 300 is initiated, at which point controltransfers to block 304. In block 304, the power control target isreceived. Next, in block 306, an assigned channel sounding burstschedule is received and the associated target transmit power spectraldensity level is scaled (based on a channel sounding bandwidthassociated with an assigned channel sounding burst) at an SS. The powercontrol target may be, for example, periodically broadcast from aserving BS on a common control channel (CCH).

Scaling of the target transmit power spectral density level facilitatesthe transmission of sounding RSs that have different bandwidths. Forexample, assuming that the associated bandwidth of the power controltarget corresponds to four resource blocks (e.g., forty-eightsubcarriers) and an assigned channel sounding burst has an associatedbandwidth of two resource blocks (e.g., twenty-four subcarriers), thenthe scaled target transmit power spectral density level would be 3 dBabove the target transmit power spectral density level indicated by thepower control target. As another example, assuming that the associatedbandwidth of the power control target correspond to four resource blocks(e.g., forty-eight subcarriers) and an assigned channel sounding bursthas an associated bandwidth of eight resource blocks (e.g., ninety-sixsubcarriers), then the scaled target transmit power spectral densitylevel would be 3 dB below the target transmit power spectral densitylevel of the power control target. Next, in block 308, the assignedchannel sounding burst is transmitted by the SS at the scaled targettransmit power spectral density level. Following block 308, controltransfers to block 310, where control returns to a calling routine.

Turning to FIG. 4, a CINR determination process 400, that may beemployed in a serving BS of a wireless communication system, isdepicted. The process 400 may be utilized to determine a CINR of atraining signal (or a non-training signal) and is initiated at block402, at which point control transfers to block 404. In block 404, theserving BS receives transmitted channel sounding symbols from multipleSSs. Next, in block 406, the received channel sounding symbols aredecoded. Then, in block 408, CINRs are determined for the SSs using thedecoded channel sounding symbols and, for example, one or more blankcyclic shifts. As noted above, a blank cyclic shift is a sequence thatwas not assigned to an SS for transmission. The blank cyclic shiftallows the serving BS to make a determination of the interference andnoise (I+N) level on a channel. In general, the interference may beattributed to SSs in adjacent cells and the noise is white noise (e.g.,thermal noise) attributable to a receiver of the serving BS. The CINRdetermined from the decoded sounding RS(s) for an SS may be combinedwith a CINR determined from a decoded demodulation RS(s) for the SS. Forexample, CINRs associated with demodulation and sounding RSs may beaveraged to provide a better indication of channel quality. As anotherexample, CINRs associated with demodulation and sounding RSs may beweighted and averaged to provide a better indication of channel quality.In this manner, the CINRs may be utilized to improve channel estimationfor data demodulation. Next, in block 410, control returns to a callingroutine.

With reference to FIG. 5, an example wireless communication system 500is depicted that includes a plurality of wireless devices (subscriberstations) 502, e.g., hand-held computers, personal digital assistants(PDAs), cellular telephones, etc., that may implement channel soundingaccording to one or more embodiments of the present disclosure. Ingeneral, the wireless devices 502 include a processor 508 (e.g., adigital signal processor (DSP)), a transceiver 506, and one or moreinput/output devices 504 (e.g., a camera, a keypad, display, etc.),among other components not shown in FIG. 5. As is noted above, accordingto various embodiments of the present disclosure, techniques aredisclosed that generally improve channel sounding for a wirelesscommunication device, such as the wireless devices 502. The wirelessdevices 502 communicate with a base station controller (BSC) 512 of abase station subsystem (BSS) 510, via one or more base transceiverstations (BTS) 514, to receive or transmit voice, data, or both voiceand data. The BSC 512 may, for example, be configured to schedulecommunications for the wireless devices 502.

The BSC 512 is also in communication with a packet control unit (PCU)516, which is in communication with a serving general packet radioservice (GPRS) support node (SGSN) 522. The SGSN 522 is in communicationwith a gateway GPRS support node (GGSN) 524, both of which are includedwithin a GPRS core network 520. The GGSN 524 provides access tocomputer(s) 526 coupled to Internet/intranet 528. In this manner, thewireless devices 502 may receive data from and/or transmit data tocomputers coupled to the Internet/intranet 528. For example, when thedevices 502 include a camera, images may be transferred to a computer526 coupled to the Internet/intranet 528 or to another one of thedevices 502. The BSC 512 is also in communication with a mobileswitching center/visitor location register (MSC/VLR) 534, which is incommunication with a home location register (HLR), an authenticationcenter (AUC), and an equipment identity register (EIR) 532. In a typicalimplementation, the MSC/VLR 534 and the HLR, AUC, and EIR 532 arelocated within a network and switching subsystem (NSS) 530, which mayalso perform scheduling for the system 500. The SGSN 522 may communicatedirectly with the HLR, AUC and EIR 532. As is also shown, the MSC/VLR534 is in communication with a public switched telephone network (PSTN)542, which facilitates communication between wireless devices 502 andland telephones 540. It should be appreciated that other types ofwireless systems, having different configurations, may implement variousaspects of the channel sounding techniques disclosed herein.

Turning to FIG. 6, an example graph provides simulated error performanceof a wireless communication system that employs conventional channelsounding (i.e., frequency hopping) with that of wireless communicationsystems that employ channel sounding according to various embodiments ofthe present disclosure. The graph plots average error vector magnitude(EVM) of CINR estimation versus signal-to-noise ratio (SNR) for threeSSs (i.e., a low-geometry SS that sounds an entire system bandwidth inone channel sounding symbol (large bandwidth w/lower PSD); a frequencyhopping low-geometry SS that sounds the entire system bandwidth overfive sounding symbols (frequency hopping w/small bandwidth); and anon-power limited low-geometry SS that sounds the entire systembandwidth in one channel sounding symbol (large bandwidth w/fixed PSD)).As is illustrated, there is essentially no channel estimationdegradation attributable to wideband channel sounding according to thepresent disclosure. This is true even for the case where a soundingbandwidth is such that power spectral density (PSD) indicates that alow-geometry SS is power-limited (i.e., large bandwidth w/lower PSD). Asis also illustrated, for a non-power limited SS (large bandwidth w/fixedPSD) the simulated error performance is significantly improved over theprior art (frequency hopping w/small bandwidth).

As mentioned above, when a power limited SS, e.g., a cell-edge SS,sounds a larger bandwidth with a lower PSD, interference in neighboringcells is reduced as the power of the sounding RS power is spread over alarger bandwidth. On the other hand, in the case of frequency hopping,when the cross-correlation between Zadoff-Chu (ZC) sequences withdifferent lengths is high, interference in neighboring cells isincreased. Moreover, frequency hopping of sounding RSs does not increasethe accuracy of channel state information (CSI) measurement. Forexample, a wireless communication with a system bandwidth of 5 MHz mayutilize 25 data resource blocks (RBs). In the frequency hopping case, asounding RS may sound 5 RBs at a time. When an SS is power-limited,sounding an entire system bandwidth (25 RBs) provides a sounding RS thathas a 7 dB lower PSD than the frequency hopping sounding RS (5RBs). Inthe simulation results depicted in FIG. 6, the average EVM of thefrequency response of the channel was used as the CSI estimate accuracyindicator. In the simulation, the frequency response was measured overthe smaller bandwidth occupied by the frequency hopping sounding RS,which is a setting that favors frequency hopping. As is illustrated inFIG. 6, even when the entire system bandwidth sounding RS has a 7 dBlower PSD than the frequency hopping sounding RS, essentially the sameCSI estimation accuracy is achieved. This response is attributable tothe fact that entire system bandwidth sounding RS has a higher spreadinggain (7 dB), which compensates for the 7 dB loss in signal strength. The7 dB spreading gain is readily observable when an SS is notpower-limited (large bandwidth w/fixed PSD). Accordingly, a number oftechniques have been disclosed herein that generally improve channelsounding in a wireless communication system.

As used herein, a software system can include one or more objects,agents, threads, subroutines, separate software applications, two ormore lines of code or other suitable software structures operating inone or more separate software applications, on one or more differentprocessors, or other suitable software architectures.

As will be appreciated, the processes in preferred embodiments of thepresent invention may be implemented using any combination of computerprogramming software, firmware or hardware. As a preparatory step topracticing the invention in software, the computer programming code(whether software or firmware) according to a preferred embodiment willtypically be stored in one or more machine readable storage mediums suchas fixed (hard) drives, diskettes, optical disks, magnetic tape,semiconductor memories such as read-only memories (ROMs), programmableROMs (PROMs), etc., thereby making an article of manufacture inaccordance with the invention. The article of manufacture containing thecomputer programming code is used by either executing the code directlyfrom the storage device, by copying the code from the storage deviceinto another storage device such as a hard disk, random access memory(RAM), etc., or by transmitting the code for remote execution. Themethod form of the invention may be practiced by combining one or moremachine-readable storage devices containing the code according to thepresent disclosure with appropriate standard computer hardware toexecute the code contained therein. An apparatus for practicing thetechniques of the present disclosure could be one or more computers andstorage systems containing or having network access to computerprogram(s) coded in accordance with the present disclosure.

Although the invention is described herein with reference to specificembodiments, various modifications and changes can be made withoutdeparting from the scope of the present invention as set forth in theclaims below. For example, the channel sounding techniques disclosedherein are generally broadly applicable to wireless communicationsystems. Accordingly, the specification and figures are to be regardedin an illustrative rather than a restrictive sense, and all suchmodifications are intended to be included with the scope of the presentinvention. Any benefits, advantages, or solution to problems that aredescribed herein with regard to specific embodiments are not intended tobe construed as a critical, required, or essential feature or element ofany or all the claims.

Unless stated otherwise, terms such as “first” and “second” are used toarbitrarily distinguish between the elements such terms describe. Thus,these terms are not necessarily intended to indicate temporal or otherprioritization of such elements.

1. A method of operating a wireless communication system, comprising:determining respective geometries of multiple subscriber stations withrespect to a serving base station, the multiple subscriber stationsincluding a first subscriber station and a second subscriber station;and scheduling, based on the determining the respective geometries,respective channel sounding bandwidths for sounding a channel betweenthe multiple subscriber stations and the serving base station, whereinthe respective channel sounding bandwidths include a first channelsounding bandwidth associated with the first subscriber station and asecond channel sounding bandwidth associated with the second subscriberstation, and wherein the first channel sounding bandwidth is greaterthan or equal to the second channel sounding bandwidth and the firstsubscriber station has a lower geometry than the second subscriberstation.
 2. The method of claim 1, wherein the scheduling furthercomprises: scheduling, based on the determining the respectivegeometries, the first channel sounding bandwidth to a first value andthe second channel sounding bandwidth to a second value, wherein thefirst value is greater than the second value.
 3. The method of claim 1,wherein the scheduling further comprises: scheduling, based on thedetermining the respective geometries, the first subscriber station tosound an entire system bandwidth in a first sounding symbol.
 4. Themethod of claim 3, wherein the scheduling further comprises: scheduling,based on the determining the respective geometries, the secondsubscriber station to sound less than the entire system bandwidth in asecond sounding symbol.
 5. The method of claim 1, wherein thedetermining respective geometries further comprises: determiningrespective carrier to interference and noise ratios for the multiplesubscriber stations, wherein the respective carrier to interference andnoise ratio of the first subscriber station is lower than the respectivecarrier to interference and noise ratio of the second subscriberstation.
 6. The method of claim 1, further comprising: assigningrespective channel sounding burst schedules to the multiple subscriberstations; and communicating, to the multiple subscriber stations, therespective channel sounding burst schedules.
 7. The method of claim 6,further comprising: receiving, from the multiple subscriber stations,respective channel sounding bursts on the channel based on therespective channel sounding burst schedules.
 8. The method of claim 7,further comprising: characterizing, for the multiple subscriberstations, the channel based on the received respective channel soundingbursts, wherein the characterizing includes determining respectivecarrier to interference and noise ratios for the multiple subscriberstations.
 9. The method of claim 7, wherein at least some of themultiple subscriber stations are assigned to a same portion of an uplinksubframe of the channel and the receiving further comprises: receiving,from each of the multiple subscriber stations that are assigned to thesame portion of the uplink subframe, respective orthogonal channelsounding symbols, included within the respective channel soundingbursts, based on the respective channel sounding burst schedules. 10.The method of claim 1, further comprising: performing data demodulationbased on received respective channel sounding bursts and receivedrespective demodulation reference signals associated with the multiplesubscriber stations.
 11. The method of claim 1, further comprising:receiving, from the first subscriber station, a first channel soundingburst transmitted at a scaled target transmit power spectral densitylevel over the first channel sounding bandwidth, wherein the scaledtarget transmit power spectral density level is based on a power controltarget that specifies a target transmit power spectral density level fora target channel sounding burst that has an associated first bandwidth,and wherein the scaled target transmit power spectral density level isbased on a relationship of the first bandwidth to the first channelsounding bandwidth.
 12. A method of operating a wireless communicationsystem, comprising: receiving, at a first subscriber station, a powercontrol target, wherein the power control target specifies a targettransmit power spectral density level for transmitting a target channelsounding burst having an associated first bandwidth from the firstsubscriber station; scaling, at the first subscriber station, the targettransmit power spectral density level based on a first channel soundingbandwidth associated with a first channel sounding burst; andtransmitting, from the first subscriber station, the first channelsounding burst at the scaled target transmit power spectral densitylevel, wherein the first channel sounding burst is a code divisionmultiplexed signal.
 13. The method of claim 12, wherein the scalingfurther comprises: decreasing the scaled target transmit power spectraldensity level when the first channel sounding bandwidth for the firstchannel sounding burst is greater than the first bandwidth.
 14. Themethod of claim 12, wherein the scaling further comprises: increasingthe scaled target transmit power spectral density level when the firstchannel sounding bandwidth for the first channel sounding burst is lessthan the first bandwidth.
 15. The method of claim 12, furthercomprising: receiving, at the first subscriber station, a channelsounding burst schedule for the first channel sounding burst, whereinthe first channel sounding bandwidth is greater than or equal to asecond channel sounding bandwidth associated with a second subscriber,and wherein the first subscriber station has a lower geometry than thesecond subscriber station.
 16. The method of claim 15, wherein the firstsubscriber station has a lower carrier to interference and noise ratiothan the second subscriber station.
 17. A wireless communication system,comprising: a base station configured to determine respective geometriesof multiple subscriber stations with respect to the base station; and ascheduler configured to set respective channel sounding bandwidths forsounding a channel between the multiple subscriber stations and the basestation based on the respective geometries of the multiple subscriberstations, wherein the respective channel sounding bandwidths of lowergeometry subscriber stations, included within the multiple subscriberstations, are greater than or equal to the respective channel soundingbandwidths of higher geometry subscriber stations, included within themultiple subscriber stations.
 18. The wireless communication system ofclaim 17, wherein the respective channel sounding bandwidths of thelower geometry subscriber stations are greater than the respectivechannel sounding bandwidths of the higher geometry subscriber stations.19. The wireless communication system of claim 17, wherein the multiplesubscriber stations are each configured to: receive a power controltarget that specifies a target transmit power spectral density level fortransmitting a target channel sounding burst having an associated firstbandwidth; scale the target transmit power spectral density level basedon a relationship of the first bandwidth to the respective channelsounding bandwidths for respective channel sounding bursts; and transmitthe respective channel sounding bursts at the scaled target transmitpower spectral density level, wherein the respective channel soundingbursts are code division multiplexed signals.
 20. The wirelesscommunication system of claim 17, wherein the base station is furtherconfigured to: receive respective channel sounding bursts from themultiple subscriber stations on the channel; and characterize, for themultiple subscriber stations, the channel based on the receivedrespective channel sounding bursts.
 21. The wireless communicationsystem of claim 20, wherein the base station is further configured toperform data demodulation based on the received respective channelsounding bursts and received respective demodulation reference signalsassociated with the multiple subscriber stations.